Metal particles which find utility as electrically and thermally conductive fillers for polymeric materials typically comprise gold, silver, copper, nickel or aluminium. Gold and silver whilst possessing excellent conductivity properties are, however, expensive. Nickel and copper display, initially, good electrical conductivities but deleteriously are susceptible to oxidation with the concomitant diminution of desirable properties such as quality and stability.
In order to overcome the problems associated with nickel and copper, namely oxidation, or the high cost of using silver, the substitution by silver coated copper particles for solid silver particles has been contemplated and methods for the production thereof explored.
In Japanese Patent Publication No. 3019 there is disclosed a method for the electroless deposition of silver on copper particles which deleteriously involves the use of potentially environmentally hazardous sodium cyanide solution.
A method for the precipitation of silver on copper powder using silver nitrate, ammonium bicarbonate and sodium ethylenediamine tetracetic acid (EDTA) is disclosed in Japanese Publication No. 59283. Although exhibiting acceptable electrical conductivity, the produced silver coated copper powder was susceptible to the effects of humidity and consequently, its oxidative stability failed to meet accepted tolerances.
Koto et al. in U.S. Pat. No. 4,652,465 describe a method for producing silver coated copper powder by initially forming a silver complex solution from a silver salt, ammonium carbonate or bicarbonate and ammonium hydroxide. The copper powder is then contacted with the silver complex solution to thereby precipitate metallic silver upon the copper particles. Whilst the method described is advantageously applicable to the use of any silver salt, the problem inherent therein resides in its ability to effect only partial coverage of silver on the copper particles.
U.S. Pat. No. 5,178,909 issued to T. Hayashi teaches the preparation of silver coated copper-based powders involving the steps of dispersing copper powders in a chelating agent solution, for example EDTA, adding a silver ion solution to the resultant dispersion to effect a substitution type of deposition reaction. This is followed by the addition of a reducing agent to obtain a reduction type of deposition reaction to thereby form silver coats on the copper powders. The chelating agent may be added in two or three portions, namely prior to the addition of the silver ion and with the addition of the silver ion and with the addition of the reducing agent or as just described but with further addition at the time of completion of the reaction. The major disadvantage of this process lies in the lack of uniform distribution of silver on the copper-based powders.
The disadvantages associated with all of the prior art processes described supra are their inability to produce silver coated copper or, copper-based, powders having uniform thickness or integral silver coverage thereof.